This invention generally relates to an apparatus for electrolytically processing metal workpieces, including zinc and tin electroplating installations. More particularly, it relates to a radial plating cell comprising a winding roll for winding a metal strip thereon and an arch-shaped anode wherein electrolytic solution passes between the winding roll and the anode.
Surface-treated steel strips are often produced by subjecting steel strips to electrolytic plating. In such electrolytic plating, abnormal deposits are sometimes formed on edge portions of a metal strip or a workpiece to be plated. Such abnormal plating phenomena will be readily understood by referring to FIG. 9. There are known two phenomena both due to local concentration of electric current as shown by arrows 56 at an edge 51' of a metal strip substrate 51. One phenomenon is known as edge overcoating wherein a deposit 54 on the metal strip edge 51' becomes thicker than a deposit 52 on an intermediate portion of the metal strip. The other phenomenon is known as edge whisker wherein plating metal deposits and grows on the metal strip edge 51' to form whiskers 55. When these phenomena take place, undesirably the metal deposit on the metal strip edge 51' tends to spall from the metal strip substrate 51. Metal spalls that have separated from the substrate will deposit on rolls or molds in the plating line or a subsequent pressing line, causing scratches or damages to the product.
Formation of such an edge overcoat 54 and edge whiskers 55 may be controlled by interposing insulating plates 18 between the anodes 16 and the metal strip substrate 51 in proximity to its edge as shown in FIG. 10, thereby preventing current from concentrating at the strip edge. These insulating plates 18 are known as edge masks. The use of edge masks is known for horizontal and vertical plating cells from Japanese Utility Model Application Kokai Nos. 58-168567, 59-21667, and 60-117863, and Japanese Patented Publication No. 53-54505.
FIGS. 11 and 12 illustrate a conventional horizontal plating cell 60 having a variety of drive means for driving a support 19 of an edge mask. In general, U-shaped edge masks 18 are inserted between the anodes 16 in proximity to the edges of a metal strip 12 by moving the support 19 back and forth through the drive means. The drive means used may be in the form of, for example, a hydraulic cylinder 62, an electrically powered cylinder 64, a rack and pinion 66, or a screw jack 68. This edge mask inserting method can be effectively applied to both horizontal and vertical plating cells.
FIGS. 13 and 14 illustrate a conventional radial plating cell 10 having a variety of drive means for driving a support 19 of an edge mask. Generally, the radial plating cell 10 includes a winding roll 14 for winding a metal strip thereon and an arch-shaped anode 16 opposed to the winding roll. The distance d between the winding roll 14 and the anode 16 must be increased in order to allow edge masks 18 to be inserted between the metal strip 12 and the anode 16, losing the advantage of the radial plating cell that a narrow gap can be kept between the metal strip 12 and the anode 16.
With the conventional arrangements illustrated, the edge masks 18 will swing or vibrate between the metal strip 12 and the anode 16. If the edge masks 18 contact the metal strip 12, both the metal strip 12 and the edge masks 18 are mechanically damaged. The supports 19 for holding the edge masks 18 must then be of high stiffness. It is difficult to increase the stiffness of the edge mask support 19 which is to be inserted in the narrow spacing between the metal strip 12 and the anode 16. The use of a thick support 19 with high stiffness will undesirably close partially the path for electrolytic solution flowing between the winding roll 14 and the anode 16. It is then difficult to maintain uniform distribution of flow velocity of plating solution between the metal strip 12 and the anode 16 in a transverse direction or a direction perpendicular to the moving direction of the metal strip.
Since drive means for moving each edge mask 18, for example, a hydraulic cylinder 62, an electrically powered cylinder 64, a rack and pinion 66, or a screw jack 68 is located outside the cell, another problem contemplated in these arrangements is a large extra space occupied by the drive means.
A prior arrangement wherein edge masks are inserted between the winding roll and the anode was proposed in Japanese Patent Application Kokai No. 58-113396.
In the plating cell according to this disclosure, edge masks are inserted between the winding roll and the anode, namely, in the spacing between two electrodes. The support for holding the edge masks are located nearer to the winding roll and fixed by guide rod and further by guide roller.
This arrangement serves to prevent the edge overcoat, but fails in narrowing the spacing between the electrodes to the desired extent, and still has unsatisfactory defects such as complicated structure of edge masks, difficulty in controlling for the movement of edge masks and tendency to causing troubles in operation.
There is thus the need for edge masks having simple structure, less problems for controlling the movement thereof and enabling safe operation for a long time.